In safety instrumentation systems of plants and so on in which a high level of safety is required, for example, fault diagnosis for a memory of a controller to control the system is required.
Since a continuous operation without restart is required for safety instrumentation systems for a long time of year units, it is necessary to perform fault diagnosis for the memory not only at the time of starting up the system but also in operation.
Generally, as faults of a memory which becomes a fault diagnostic object, in addition to a read and write (Read/Write) error of a memory cell, there is a coupling fault that when a certain memory cell is Read/Written the value of another memory cell changes.
As fault diagnostic algorithms for this coupling fault, there are Walkpath (called also Walking Bit) and GALPAT.
Walkpath is a method which tests, while the value of a focused memory cell of a bit or a plurality of bits is turned ON/OFF across the whole memory area of a diagnostic object, whether or not the values of the remaining memory area are correct.
Though this method can detect a coupling fault in the whole memory area, since the number of times of read and write (Read/Write) of the memory is proportional to the square of the memory size, there is a problem that a fault diagnostic time increases at an accelerating speed if the capacity of a memory of a diagnosis object becomes large.
For this problem, as a method to shorten the processing time required for memory fault diagnosis, there is a method to hierarchically set a fault diagnosis area of the memory to thereby reduce the number of memory access times (refer to Japanese Patent Publication No. 4312818, for example).